feat(photos): add 1h disk cache for remote thumbnails and keep tabs mounted

Closes #686

- Add trekPhotoCache service: SHA1-keyed disk cache under uploads/photos/trek/,
  1h TTL, in-flight dedup map to prevent stampedes on concurrent requests
- Add migration 108: trek_photo_cache_meta table
- Hook cache into streamPhoto for Immich/Synology thumbnail path;
  originals bypass cache
- Add fetchImmichThumbnailBytes / fetchSynologyThumbnailBytes returning
  Buffer instead of piping, used by the cache layer
- Add scheduler entry (every 2h + startup sweep) to evict expired disk
  files and DB rows via sweepExpired()
- Client: convert journey tab conditional-mount to hidden-toggle so
  img elements stay in DOM across tab switches, preventing redundant
  thumbnail requests on rapid tab changes
- Expose invalidateSize() on JourneyMapHandle; call it on map tab
  activation to fix Leaflet rendering in previously-hidden container

server/src/scheduler.ts

@@ -248,12 +248,36 @@ function startIdempotencyCleanup(): void {
   }, { timezone: tz });
 }
 
+// Trek photo cache cleanup: every 2 hours — evict disk files and DB rows past their 1h TTL
+let trekPhotoCacheTask: ScheduledTask | null = null;
+
+function startTrekPhotoCacheCleanup(): void {
+  if (trekPhotoCacheTask) { trekPhotoCacheTask.stop(); trekPhotoCacheTask = null; }
+
+  // Run once immediately on startup to evict any entries left over from a previous run
+  try {
+    const { sweepExpired } = require('./services/memories/trekPhotoCache');
+    sweepExpired();
+  } catch { /* cache dir may not exist yet — harmless */ }
+
+  trekPhotoCacheTask = cron.schedule('0 */2 * * *', () => {
+    try {
+      const { sweepExpired } = require('./services/memories/trekPhotoCache');
+      sweepExpired();
+    } catch (err: unknown) {
+      const { logError: le } = require('./services/auditLog');
+      le(`Trek photo cache cleanup: ${err instanceof Error ? err.message : err}`);
+    }
+  });
+}
+
 function stop(): void {
   if (currentTask) { currentTask.stop(); currentTask = null; }
   if (demoTask) { demoTask.stop(); demoTask = null; }
   if (reminderTask) { reminderTask.stop(); reminderTask = null; }
   if (versionCheckTask) { versionCheckTask.stop(); versionCheckTask = null; }
   if (idempotencyCleanupTask) { idempotencyCleanupTask.stop(); idempotencyCleanupTask = null; }
+  if (trekPhotoCacheTask) { trekPhotoCacheTask.stop(); trekPhotoCacheTask = null; }
 }
 
-export { start, stop, startDemoReset, startTripReminders, startVersionCheck, startIdempotencyCleanup, loadSettings, saveSettings, VALID_INTERVALS };
+export { start, stop, startDemoReset, startTripReminders, startVersionCheck, startIdempotencyCleanup, startTrekPhotoCacheCleanup, loadSettings, saveSettings, VALID_INTERVALS };